1. Field of the Invention
This invention relates to a phased array transducer. In particular, this invention relates to a transducer which may be used for emitting and/or receiving ultrasound. Still more particularly, this invention relates to an ultrasonic transducer for transducing ultrasound of a predetermined frequency. The transducer is of the type which contains a large number of piezoelectric elements having all the same natural frequency. Each of these elements has a first and a second surface which are located opposite to each other, and each has a first and a second electrode which are provided on the first and the second surface, respectively. Still more particularly, the invention relates to an ultrasonic transducer for medical purposes, especially for medical ultrasound examinations.
2. Description of the Prior Art
In U.S. Pat. Nos. 4,211,948 and 4,211,949, for instance, is disclosed a phased array transducer for emitting and receiving ultrasound. The transducer contains a large number of elongated piezoelectric elements which are arranged parallel to each other. All piezoelectric elements have the same natural (mechanical) frequency. Conventionally, the piezoelectric elements contain a first and a second surface which are arranged parallel to each other. Each of these surfaces is provided with an electrode or metallic coating for connecting an electrical actuation circuit and/or an electrical evaluation circuit thereto. The depth of the piezoelectric elements is chosen such that their natural frequency is in a certain relationship to the predetermined frequency applied to the electrodes. In particular, the depth or thickness between the metallic coatings is chosen to be one half wavelength at the emission frequency. Thus, each piezoelectric element is a half wave resonator.
One of the problems associated with the prior art ultrasonic transducer resides in the fact that production becomes more difficult the thicker and narrower the individual elements must be in order to meet the requirements of frequency adjustment. In the prior art transducer arrays, the piezoelectric material tends to break during manufacture.
Another problem associated with the prior art transducer resides in the conventional resonance excitation. In a phased array arrangement the individual elements have all the same natural frequency. Since the elements are usually acoustically coupled together via a matching layer and backing, excitation of one element at resonance leads to a resonant unwanted excitation of the elements nearby. This may bring about undesired electrical and acoustic signals.